primary compressed air gauge is reading 120 PSI, and the secondary is reading 102 PSI. When the compressor turns on the secondary gauge won't go past 110 PSI.
Do you know what the main issue might be causing it?
First build up the air till you get the max you can on the gauges. Then shut the truck off and check for leaks at the front of the truck. First with the parking brake applied and then released. The secondary system is for the front brakes. If no leaks, then check the line from the air dryer to the front axle reservoir. You may have a bad gauge or a leak in the line between the tank and the gauge. If there is enough room you could try to swap the lines between the gauges and then see what they read.